Sexual Selection: Evolution Gets Intimate

This article was originally written for the Wellcome Trust science writing competition 2012. Do get in touch if you would like to feature this article, or something similar, in your own publication.

Darwin once said: “The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me feel sick.” And he had a point. Have you ever looked at a peacock – really looked at one – and wondered how such bizarre and impractical plumage could even exist? Surely, according to Darwin’s theory of evolution, natural selection should have driven it to extinction by now.

So what’s going on here?

The answer is a kind of natural selection that Darwin had never foreseen: sexual selection. Here’s how it works.

Bearing young takes a lot of energy, so in most species, the females are the picky ones. If a male evolves a trait that females find attractive, he will get more action than all the other males, and pass on his “sexy” feature to the rest of the population, regardless of whether the trait actually aids survival.

Females might find a trait attractive for several reasons. North Island robins are programmed to seek out red berries on trees, so females might associate a male’s red markings with food.

Alternatively, an impractical “ornament” like a peacock’s tail can act as an honest display of a male’s fitness. If a male is still able to feed and escape predators despite this handicap, it proves he is very good at surviving. A peacock’s tail is effectively crying out: “Look at me! Only an incredibly strapping and capable male can manage to survive while dragging this ridiculous tail around. You definitely want to have sex with me so you can have strapping and capable babies!”

In fact, the growth of male ornaments, such as the comb on a Red Junglefowl (the ancestor of our domesticated chicken), requires large quantities of testosterone, a hormone that actually suppresses the immune system. Remaining disease-free in the face of such difficulty is the sign of a truly good match. And humans are not excluded from this. Studies have shown that muscular men are more prone to catching minor infections like colds than skinny men, because high testosterone suppresses the immune system. So at least being skinny has some perks.

But sexual selection needn’t have anything to do with a male’s strength. Occasionally, a female may simply evolve a random preference for something that may not even exist yet. The gene for this preference can spread through the population, but may never come into action – unless a mutation occurs in one lucky male, which makes him the object of their desire.

This might all sound a bit mad, but believe me, it happens. Let me tell you a tale of two fish: the swordtail and the platyfish.

The swordtail, unsurprisingly, has a tail shaped like a sword. The platyfish (an “older” species that was earlier to branch off the evolutionary tree) does not. Intriguingly though, the platyfish does have a preference for sword-shaped tails: a preference for something that doesn’t exist in their own species.

To discover this, scientists used the most cutting edge and advanced techniques: they stuck artificial swords to male platyfish’s tails. The effect on females was impressive. When it came to sheer sex appeal, platyfish with artificial swords won hands down. This implies that a preference for sword-shaped tails was lurking around long before the sword shape ever appeared.

This raises an interesting question when it comes to humans: what if our populations are rife with hidden preferences for sexual characteristics that haven’t evolved yet, and maybe never will? What if we are all concealing these secret desires, waiting for the right trait to appear so they can finally be fulfilled?

Or what if they are already being fulfilled? Humans are able to modify their surroundings, and themselves, to a far greater extent than many animals. Right up until the 1930s, the people of Papua New Guinea bound their children’s heads into an elongated shape that was seen as being the height of beauty. And sexual ornaments aren’t just exclusive to males – what if Chinese foot binding, Victorian corsets, even today’s towering stilettos, are simply satisfying a fetish for a physical trait that hasn’t evolved yet? If this were the case, it would certainly explain my attraction to boys in pointy shoes.

But sexual ornaments, like 1960s fashions, become outmoded. A trait that is successful with the females will spread through the whole population – but once everyone has it, it’s not special any more. That’s why new ornaments will evolve to make some males stand out in the crowd. The problem is that all the old ornaments still need to remain, as these now make up the female’s mental image of what a male should look like. And so we return to the tragic picture of the peacock, weighed down by countless ostentatious ornaments, only a few of which are still even noticed by the females. No wonder Darwin felt sick.